Aircraft commonly use hydraulic systems to facilitate control of their movement through the air. That is, hydraulic systems respond to controls in the cockpit so as to cause flight control surfaces to move in a manner that results in desired changes in the attitude and heading of the aircraft.
Although redundant hydraulic systems are commonly utilized, there are still occasions when a complete failure of all of the hydraulic systems renders the flight control surfaces uncontrollable. For example, such a complete failure may occur in the event of a terrorist attack. A shoulder fired missile or an onboard bomb may completely disable an aircraft's hydraulic systems.
In such instances, it is known to use engine thrust in an attempt to control the attitude and heading of the aircraft. However, according to contemporary practice, this is accomplished by carefully manipulating the aircraft's throttles.
Use of the aircraft's throttles to control the attitude and heading of the aircraft is inherently difficult and limited in effectiveness. Use of the aircraft's throttles to control attitude and heading requires substantial skill. However, even when performed by a skillful pilot, manipulation of the throttles does not always provide the desire changes in attitude and heading.
Indeed, sometimes manipulation of the throttles results in undesirable changes in attitude and heading. Some of the manipulations required for aircraft control are counterintuitive and are therefore not easily practiced by a human operator. This is particularly true for aircraft that are inherently unstable and/or require command limiting and thus typically utilize fly-by-wire control augmentation.
As a result, there is a need for an aircraft backup control system that does not rely upon the skill of a pilot to use engine thrust so as to control the aircraft's attitude and heading.